Microquake seismic interferometry with SVD-enhanced
Green's function recovery
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Citation
Melo, Gabriela, and Alison Malcolm. “Microquake Seismic
Interferometry with SVD-enhanced Green’s Function Recovery.”
The Leading Edge 30.5 (2011): 556.
As Published
http://dx.doi.org/10.1190/1.3589114
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Society of Exploration Geophysicists
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Author's final manuscript
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Thu May 26 20:23:36 EDT 2016
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http://hdl.handle.net/1721.1/73636
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Microquake seismic interferometry with SV D enhanced
Green’s function recovery
Gabriela Melo and A lison Malcolm
Earth Resources Laboratory - Earth, Atmospheric, and Planetary Sciences Department
Massachusetts Institute of Technology
Abstract
The conditions under which seismic interferometry (SI) leads to the exact Green’s function (G F) are rarely met in practice, resulting in errors in the recovered G F. To alleviate this
problem, we employ additional information than what is typically used in SI. This information
comes from the collection of crosscorrelated traces, one for each source for a pair of receivers,
which we shall refer to as the crosscorrelogram. It is by stacking the crosscorrelogram in
the source dimension that we obtain an interferometric G F. In general, this crosscorrelogram
has both stationary energy that contributes to the estimated G F and non-stationary energy that
does not. Stationary energy in the crosscorrelogram is characterized by linearity, coherency,
low wavenumber, and thus nearly in-phase events along the source dimension. Non-stationary
energy by contrast is characterized by non-linearity, incoherency, high wavenumber, and outof-phase events along the source dimension. We exploit these differences to separate the two
parts of the energy in the crosscorrelogram to obtain more accurate G F estimates for non-ideal
cases.
In order to perform this separation and extract more information from the crosscorrelograms we use the singular value decomposition (SV D). We find that SV D is able to enhance
physical arrivals that are not properly recovered using standard stacking in SI and in many cases
to recover arrivals that would otherwise be obscured by noise. Here, we filter the crosscorrelograms by using a lower-rank approximation, computed with SV D by keeping only the largest
singular values, to enhance events that are coherent across multiple sources, thus isolating this
stationary energy that gives the primary contribution to the G F. We illustrate this method with
synthetic results for both homogeneous and scattering media simulating a possible application
in microseismic monitoring with downhole receivers.
INTRODUCTION
The conditions under which seismic interferometry (SI) leads to the exact Green’s function (G F)
are rarely met in practice. As a result, we generally recover only estimates of the true G F. This
raises the questions: How good an approximation to the G F can SI give? Can we improve this
estimated G F?
To recover the full G F between two receivers using SI requires that these two receivers be
surrounded by a surface of sources, with both monopole and dipole sources required for accurate
amplitude estimates. Since dipole sources are rarely available in practice, here we focus primarily
on recovering traveltimes. Accurate estimation of these traveltimes still requires full (monopole)
source coverage, however, an assumption that is rarely met in practice. This results in a degradation
of the quality of the recovered G F, which then needs to be carefully interpreted. In the ideal case,
Snieder (2004) showed that the sources that give the main contribution to the causal and anti-causal
G Fs are the ones located along the ray path between the two receivers, and those in the Fresnel
zone around these sources. Snieder came to this conclusion using the method of stationary phase;
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the sources along the raypath are the sources at which the phase is stationary. Energy emanated by
sources outside the Fresnel zone should cancel out, again assuming full source coverage, as they
are outside the zone in which the phase is stationary.
When the coverage is not ideal, or when the source/receiver locations and raypaths are not well
known this non-stationary energy will not cancel resulting in errors in the recovered G F. To alleviate this problem, we employ additional information than is typically used. This information comes
from the collection of crosscorrelated traces, one for each source for a pair of receivers, which we
shall refer to as the crosscorrelogram. It is by stacking the crosscorrelogram in the source dimension that we obtain an interferometric G F. In general, this crosscorrelogram has both stationary
energy, that should contribute to the estimated G F and non-stationary energy that should not. Stationary energy in the crosscorrelogram is characterized by linearity, coherency, low wavenumber,
and thus nearly in-phase events along the source dimension. Non-stationary energy by contrast
is characterized by non-linearity, incoherency, high wavenumber, and out-of-phase events along
the source dimension. We exploit these differences to separate the two parts of the energy in the
crosscorrelogram to obtain more accurate G F estimations for non-ideal cases.
In order to perform this separation and extract more information from the crosscorrelograms to
ultimately improve the G F, we follow Melo et al. (2010) in which the singular value decomposition
(SV D) (see e.g. Golub and van Loan (1996)) is used to do this separation. SV D is a numerical
technique commonly used in seismic data processing (see e.g. Ulrych et al. (1988); Sacchi et al.
(1998)), to increase the signal to noise ratio and filter linear events. Melo et al. (2010) showed how
SV D is able to enhance physical arrivals that are not properly recovered using standard stacking
in SI and generally recover arrivals that would otherwise be obscured by noise. Here we further
investigate the relationship between SV D and SI in the microseismic context discussed below.
To understand why SV D is able to separate stationary and non-stationary energy we must first
understand the relationship between frequency and singular values. This relationship is discussed
by Hansen et al. (2006) where they explain the relationship between singular values and frequency
(or source-wavenumber in our case) - large singular values correspond to low frequencies and small
singular values correspond to high frequencies. As they correspond to low-frequencies, large singular values are associated with events that are in phase in the crosscorrelogram: stationary sources
whose energy contribute to the G F. In the context of waveguides, Philippe et al. (2008) exploit the
connection between singular values and frequency for characterization of targets. They show that
the first singular value associated with a given target is proportional to the backscattering form
function of the target, and that the second singular value is proportional to the second derivative
of the angular form function. Then, they use SV D to extract the backscattered frequency signature of a target in a waveguide. Here, we filter the crosscorrelograms by using a lower-rank
approximation, computed with SV D using the largest singular values, to enhance events that are
coherent across multiple sources, thus isolating this stationary energy. In this way, we exploit the
fact that stationary signal is at lower wavenumbers than non-stationary signal to separate it from
non-stationary signal. We illustrate this method with synthetic results for both homogeneous and
scattering media simulating a possible application with downhole receivers.
These examples are meant to illustrate the particular application we have in mind for this technique, which is the estimation of the G F between two sources in a geothermal reservoir. While
most applications of SI estimate the G F between two receivers surrounded by sources, Curtis et al.
(2009) show, using reciprocity, that it is also possible to use SI to estimate the G F between a pair
of sources. In the microseismic context this would greatly increase the available data set as there
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are generally few receivers and many sources. In addition, as is well known, noise is a major issue
with microseismic data. Noise contaminated data lead to poor event locations, which creates uncertainty as to which receivers are in the Fresnel zone for a given pair of sources. Simply summing
the responses from all receivers will not solve this problem because the receiver array is generally
sparse. Here we show how using SV D to decompose the crosscorrelogram before stacking helps
to alleviate this problem. In addition to these properties, we find that SV D also allows some level
of separation of the G F into different components - main arrivals (direct, singly reflected, and refracted waves), multiple scattering, and noise. To separate signal from noise directly in the G F is
difficult, especially for coda waves, because the noise may have comparable amplitude and temporal frequency content to the coda. Doing this separation is important because coda waves contain
information about the inhomogeneities in the medium, while noise does not. We show preliminary
results illustrating that it may be possible to extract information about these different components
in the crosscorrelated traces before stacking them to form the G F.
METHOD
We now consider the crosscorrelogram as a matrix, C , where each row is the crosscorrelation of
the signals at the two receivers from one source. Thus, the vertical dimension of C is source and
the horizontal is time, as shown in figure 1.
C
sources
stack to form GF
s1
sn
t = −tmax
t=0
t = tmax
time
Figure 1: Crosscorrelogram matrix C . Stacking over sources gives interferometric G F.
Next, we decompose the crosscorrelogram using SV D (see e.g. Golub and van Loan (1996) for
a description of SV D). The SV D decomposition of the crosscorrelogram matrix is, C = U V t ,
where U and V are the left and right singular vector matrices, and is the diagonal matrix whose
elements are the singular values of C . Figure 2 shows how we obtain a lower-rank approximation
C of the crosscorrelogram by selecting only the largest singular values of the SV D decomposition
of C . Stacking the rows of C gives the standard interferometric G F, G , and stacking the rows of
the approximation C gives the modified interferometric G F, G j , where j is the rank of C (the
number of singular values retained). In the examples that follow, we compare these two G Fs.
We now illustrate this procedure with a simple example. The model for this example is a
constant velocity and density model with no reflectors, so the G F consists of the direct wave only.
We examine how well we can approximate the true G F in three cases: (i) the case where there
are stationary sources only, (ii) non-stationary sources only, and (iii) both stationary and nonstationary. In all three cases there are gaps in the source coverage and all the G F are normalized as
we do not have dipole sources.
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Figure 2:
SV D.
C
U
Σ
Vt
C’
U
Σ’
Vt
Crosscorrelogram matrix C and its lower-rank approximation C obtained through
First, consider a case where there are sources only in the stationary-phase zone, figure 3(a).
The energy from these sources contributes constructively to the G F. Comparing the standard, figure 3(b), and rank-2, figure 3(c), crosscorrelograms and the respective estimated G Fs, G and G 2 ,
figure 3(d)-(e), we see that the standard and the lower-rank crosscorrelograms and G Fs are quite
similar. We use a rank-2 approximation of the crosscorrelogram because there are two stationaryphase zones in the crosscorrelogram and thus two signals we wish to reconstruct. In this simple
example, it is obvious what the rank of the crosscorrelogram approximation should be, which is
not the case in general. This is a case where standard interferometry works well and the SV D
technique is not necessary, although it is also not detrimental.
In case (ii) there are only non-stationary sources, figure 4(a). Ideally, all of this non-stationary
energy should cancel but if there are gaps in the source coverage residual energy will remain
because of edge effects. As is clear in figure 4(d), G is not a good estimate of the correct G F,
but it appears as if it contains a physical arrival. While G , figure 4(d), contains two non-physical
arrivals due to edge effects, G 2 , figure 4(e), does not. The rank-2 crosscorrelogram, figure 4(c), in
this case does not enhance any linearity and does not even resemble the original crosscorrelogram,
figure 4(b). The rank-2 crosscorrelogram and G 2 thus act as a diagnostic of non-physical arrivals.
Case (iii) mixes the two previous cases. Figure 5(a) shows sources uniformly distributed in each
stationary zone and each non-stationary zone, but with gaps in between. The crosscorrelogram,
figure 5(b)-(c), thus has energy contributing to the G F and energy that should cancel out completely
but, because of the gaps, does not. The rank-2 approximation filters the pseudo-noise caused by
the imperfect cancellation of non-stationary energy, and G 2 is more accurate then G as seen in
figures 5(d)-(e).
EXAMPLES
This example approximately mimics an idealized source/receiver geometry of a downhole monitoring of microseismic activity in a geothermal reservoir. We use a single borehole with 35 receivers
and estimate the G F between two micro-quakes as shown in both figure 6(a) and figure 7(a). The
reference and interferometric G Fs shown here are all normalized. The idea is to obtain the interfer4
Depth (km)
Source−receiver geometry
50
Interferometric GF
0
Reference GF
−50
−50
0
50
Distance (km)
Crosscorrelogram
Rank−2 crosscorrelogram
0
Sources
Sources
0
200
400
600
−10
−5
0
Time (s)
200
400
600
−10
5
Interferometric GF − G
0
−1
−10
0
5
Time (s)
Interferometric GF − G2
1
Amplitude
Amplitude
1
−5
−5
0
Time (s)
5
0
−1
−10
10
−5
0
Time (s)
5
10
Figure 3: (a) source-receiver geometry with 13 evenly distributed sources (red stars) in each of the
stationary zones of the receivers (blue triangles); (b) original crosscorrelogram; (c) rank-2 crosscorrelogram; (d) standard interferometric G F, G ; (e) rank-2 G F, G 2 . The black line corresponds to
the interferometric G Fs and the red line to the true G F. The G F in (d) and (e) are similar.
5
Depth (km)
Source−receiver geometry
50
Interferometric GF
0
Reference GF
−50
−50
0
50
Distance (km)
Crosscorrelogram
Rank−2 crosscorrelogram
0
Sources
Sources
0
200
400
600
−10
−5
0
Time (s)
200
400
600
−10
5
Interferometric GF − G
0
−1
−10
0
5
Time (s)
Interferometric GF − G2
1
Amplitude
Amplitude
1
−5
−5
0
Time (s)
5
0
−1
−10
10
−5
0
Time (s)
5
10
Figure 4: (a) source-receiver geometry with 21 evenly distributed sources (red stars) in each
of the non-stationary zones of the receivers (blue triangles); (b) original crosscorrelogram; (c)
rank-2 crosscorrelogram; (d) standard interferometric G F, G ; (e) rank-2 G F, G 2 . The black line
corresponds to the interferometric G Fs and the red line to the true G F. In (e) the G F does not
contain the edge effect present in (d).
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Depth (km)
Source−receiver geometry
50
Interferometric GF
0
Reference GF
−50
−50
0
50
Distance (km)
Crosscorrelogram
Rank−2 crosscorrelogram
0
Sources
Sources
0
200
400
600
−10
−5
0
Time (s)
200
400
600
−10
5
Interferometric GF − G
0
−1
−10
0
5
Time (s)
Interferometric GF − G2
1
Amplitude
Amplitude
1
−5
−5
0
Time (s)
5
0
−1
−10
10
−5
0
Time (s)
5
10
Figure 5: (a) source-receiver geometry with 13 and 9 evenly distributed sources (red stars) in
each of the stationary and non-stationary zones of the receivers (blue triangles), respectively. (b)
original crosscorrelogram; (c) rank-2 crosscorrelogram; (d) standard interferometric G F, G ; (e)
rank-2 G F, G 2 . The black line corresponds to the interferometric G Fs and the red line to the true
G F. In (e) the fluctuations are reduced and the G F is clearer than in (d).
7
ometric G F between a pair of sources (micro-quakes) instead of a pair of receivers, similar to what
is done in Curtis et al. (2009) for a larger scale problem. The medium is weakly scattering with a
constant background velocity and density. We study two cases: first, we do the crosscorrelations
in a clean dataset, figure 6, and second in a noisy dataset, figure 7. The additive noise and random scattering we use here are realizations of a Gaussian random field with prescribed correlation
lengths along given directions. We focus our observations on two things: the phase of the direct
wave and the energy in the coda. We added enough noise to completely obscure the direct wave
and distort the waveform of the coda wave in the interferometric G F and show how SV D improves
both of these measures.
In the first example, figure 6, even though the data are noise free, there is enough non-stationary
energy in the crosscorrelogram from receivers outside the Fresnel zone, figure 6(b), to create highamplitude fluctuations that hide the direct wave. Here we chose the rank-1 approximation, figure 6(c), because the G F consists only of one direct wave and thus there is only one zone of
stationary-phase energy in the crosscorrelogram. In figure 6(e) the fluctuations were damped in G 1
as compared with G in figure 6(d), leading to a much clearer G F. Crosscorrelation of both G Fs, G
and G 1 , with the reference G F peaks at 0.2835 and 0.0020 s (time sampling interval is 0.0005 s),
respectively, showing a significantly more accurate phase estimate with G 1 than with G . We use
the L 2 -norm (square-root of sum of squares) of the coda waves as a measure of the energy in the
coda. Comparing the reference and the interferometric coda waves, we find relative errors of 98%
for the coda wave in G and only 12% for G 1 , demonstrating that G 1 is a better approximating of
the true G F than G, in this norm.
To make this example more realistic, we add weakly correlated noise to the data in figure 6.
In figure 7(d) G appears strongly contaminated by noise and neither the direct arrival nor the
coda wave are visible. In figure 7(e) fluctuations and random noise are strongly attenuated in G 1 ,
revealing not only the direct arrival but also reducing the noise close to the coda wave level. The
phase differences between G and G 1 and the reference G F are 0.2850 and 0.0015 s, respectively.
The relative errors in the L 2 -norm of the coda are 148% for G and 11% for G 1 . We see that SV D
eliminates most of the noise in the coda wave, as well as the fluctuations before the direct wave,
demonstrating its stability with respect to noise.
For the noisy case, we performed tests for a variety of receiver apertures, noise levels, spacing
between receivers. We find that the absolute improvement obtained through SV D varies from case
to case but the phase of direct wave and the coda energy, in general, are closer to correct with SV D
than without. This noise attenuation is particularly important in microseismic studies as the data
are typically quite noisy. Stability with respect to aperture is also important because errors in the
location of micro-quakes can be significant.
Discussions, conclusions, and future work
The accurate estimation of the G F with non-ideal source coverage remains a significant problem
in SI. We have shown how using SV D to approximate crosscorrelograms before stacking is a
promising approach to alleviate this problem. In general, for the SV D technique to work there
must be more stationary energy than non-stationary energy in the crosscorrelogram, although this
requirement can be relaxed somewhat through normalization of the traces in the crosscorrelogram.
How much more energy is necessary and how much noise can be accommodated are subjects of
ongoing research.
8
Depth (km)
Source−receiver geometry
0
2
4
Interferometric GF
6
Reference GF
8
−5
0
5
Distance (km)
Rank−1 crosscorrelogram
10
Sources
Sources
Crosscorrelogram
20
30
10
20
30
0.4
0.6
0.8
1
Time (s)
1.2
1.4
0.4
Interferometric GF − G
0
−1
0.8
1
1.2
Time (s)
Interferometric GF − G1
1.4
1
Amplitude
Amplitude
1
0.6
0.4
0.6
0.8
1
Time (s)
1.2
0
−1
1.4
0.4
0.6
0.8
1
Time (s)
1.2
1.4
Figure 6: (a) source-receiver geometry: one borehole with 35 receivers (blue triangles) and two
micro-quakes (red stars); (b) original crosscorrelogram; (c) rank-1 crosscorrelogram; (d) standard
interferometric G F, G ; (e) rank-1 G F, G 1 . The black line corresponds to the interferometric G Fs
and the red line to the true G F. In (e) the fluctuations are reduced and the G F is clearer than in (d).
9
Depth (km)
Source−receiver geometry
0
2
4
Interferometric GF
6
Reference GF
8
−5
0
5
Distance (km)
Rank−1 crosscorrelogram
10
Sources
Sources
Crosscorrelogram
20
30
10
20
30
0.4
0.6
0.8
1
Time (s)
1.2
1.4
0.4
Interferometric GF − G
0
−1
0.8
1
1.2
Time (s)
Interferometric GF − G1
1.4
1
Amplitude
Amplitude
1
0.6
0.4
0.6
0.8
1
Time (s)
1.2
0
−1
1.4
0.4
0.6
0.8
1
Time (s)
1.2
1.4
Figure 7: (a) source-receiver geometry: one borehole with 35 receivers (blue triangles) and two
micro-quakes (red stars); (b) original crosscorrelogram; (c) rank-1 crosscorrelogram; (d) standard
interferometric G F, G ; (e) rank-1 G F, G 1 . The black line corresponds to the interferometric G Fs
and the red line to the true G F. In (e) the random noise is reduced to the level of the coda and the
G F is clearer than in (d).
10
We are also continuing to investigate which properties of coda waves can be accurately inferred from the G F obtained through SV D; the preliminary results shown here indicate that such
properties are better recovered with SV D than stacking alone. Separating real signal from noise
would lead to a coda that truly reflects the scattering characteristics of the medium thus allowing
for the use of coda waves to retrieve information about the scattering strength and through this
about fracture characteristics in a reservoir.
Acknowledgments
We would like to thank Oleg Poliannikov, Pierre Gouedard, Bongani Mashele, Huajian Yao, and
Michael Fehler for the great imputs during our discussions. This work is supported by grants from
the Department of Energy (D O E) and Chevron, and the founding members consortium at Earth
Resources Laboratory (ERL).
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